Small fluctuation of coronal X-ray intensity and a signature of nanoflares

If small energy events (nanoflares) contribute to the heating of the solar corona, they may be too small to be recognized as independent events. We cr eate a simple histogram of the X-ray intensity fluctuation around the mean intensity and find that the histogram consists of a central Gaussian compon ent and a wing component. The Gaussian component corresponds to random fluc tuation around the mean intensity. The width of this component becomes broa der with increasing intensity and is larger than the predicted photon noise distribution. We suggest that nanoflares produce the observed fluctuation of the X-ray intensity. The energy of nanoflares is estimated to be less th an 10(22) ergs to explain the observed fluctuation for active regions. It i s estimated that if the energy of nanoflares is 10(20) ergs, then these nan oflares must occur at a rate of 10(6) s(-1) in a single active region (simi lar to 10(20) cm(2)) to supply the required energy (similar to5 x 10(7) erg s cm(-2) s(-1)) to maintain the corona.